1. Field of the Invention
The present invention relates to an electron beam drawing method suitable for forming a fine pattern on a resist on a semiconductor substrate surface by an electron beam and more particularly, to an electron beam drawing method in which patterns of substantially the same size can be formed on a resist on a semiconductor substrate in a cell projection manner enabling a high speed formation of a fine pattern and in a conventional variably shaped beam manner.
2. Description of the Related Art
In company with progress of LSI, miniaturization of a pattern, which is used for a semiconductor device fabrication process, has also been rapidly advanced. A drawing method utilizing an electron beam is effective to cope with a future requirement for a width of a pattern as small as 0.25 .mu.m or less in fabrication of a semiconductor device. FIG. 1A is a schematic drawing showing a conventional electron beam exposure system and FIG. 1B is a schematic drawing showing a beam path passing through an aperture. It should be noted that hatching is made on a drawn latent image in FIG. 1B. In the electron beam exposure system, a sample stage 92, on which a semiconductor wafer 91 coated with a resist on its surface is mounted, is disposed and an electron gun 81 is disposed above the sample stage 92 which gun generates an electron beam 100 being irradiated on the semiconductor wafer 91. There are provided, between the electron gun 81 and the sample stage 92 in the sequential order from the top, a blanking electrode 82, which controls ON/OFF of irradiation of the electron beam 100 to the semiconductor wafer 91, a first aperture 83, which has an opening 83a of a rectangular shape for transforming the electron beam 100 to an electron beam 100a of a rectangular shape in section, a beam shaping lens 84, which suppresses spreading of the electron beam 100a which has passed through the first aperture 83, a shaping deflector 85, which deflects the electron beam 110a, a second aperture 86, which has an opening for variably shaped beam drawing 86f of a rectangular shape and plural openings for cell projection drawing 86a to 86e for transforming the electron beam 100a in section to a cell projection beam 100b or a variably shaped beam 100c, a demagnifying lens 87, which suppresses spreading the cell projection beam 100b and the variably shaped beam 100c which have passed through the second aperture 86, a main deflector 88 and an auxiliary deflector 89, which deflect the cell projection beam 100b and the variably shaped beam 100c, and a projection lens 90, which controls focuses of the cell projection beam 100b and the variably shaped beam 100c.
The openings for cell projection drawing 86a to 86e have different shape from one another. In FIG. 1B, the cell projection beam 100b is an electron beam after the electron beam 100a has passed through the opening 86c.
A controlling unit 96, which controls the blanking electrode 82, the shaping deflector 85, the main deflector 88 and the auxiliary deflector 89, is connected to those portions. Besides, there are connected to the controlling unit 96, a calculator 94, in which processing such as expansion of data, sorting thereof and the like through a data bus 93 are conducted and an intensity of an electron beam is calculated, a storage unit 95, in which a graphical data to be drawn on the resist on the semiconductor wafer 91 is stored, and a graphical data memory 97, which temporarily stores the graphical data.
In an electron beam exposure system constituted in such a manner, the graphical data to be drawn on the resist on the semiconductor 91 is stored in the storage unit 95 and necessary processing such as expansion of data, sorting thereof and the like is performed in the calculator 94. At this point, a part of the processing result is temporarily stored in the graphical data memory 97 and is read out therefrom. The result of processing by the calculator 94 is transmitted to controlling unit 96 and the blanking electrode 82, the shaping deflector 85, the main deflector 88 and the auxiliary deflector 89 are controlled by the controlling unit 96. Thereby, the cell projection beam 100b or the variably shaped beam 100c of a desired shape can be irradiated to a desired position on the surface of the semiconductor substrate 91.
One or more patterns are transferred to form latent images in one shot of exposure by irradiating the cell projection beam 100b having plural patterns on the resist on the surface of the semiconductor wafer 91 by use of the above mentioned electron beam exposure system. Thereby, the throughput can be improved. A sectional area of the variably shaped beam 100c is determined by a degree of superposition between the opening 83a of the first aperture 83 and the opening for variably shaped beam drawing 86f of the second aperture 86. Thus, a pattern of an arbitrary sectional area can be formed as a latent image on the resist coated on the semiconductor wafer 91.
For example, an exposure process in fabricating a Dynamic Random Access Memory (DRAM) will be described. FIG. 2 is a schematic drawing showing a structure of DRAM. DRAM is constructed with a memory cell array section 101, in which the same patterns in shape are disposed in a repeated manner and a peripheral circuit section 102, in which patterns are disposed in an irregular manner.
In the case where such a pattern of DRAM is exposed by use of the electron beams exposure system, the opening for variably shaped beam drawing 86f is selected to form the peripheral circuit section 102 and drawing in the variable shaped beam manner is performed. On the other hand, drawing in the cell projection manner is performed by selecting the opening for cell projection drawing 86a, 86b, 86c, 86d or 86e for formation of the memory cell array section 101.
In a conventional electron beam drawing method, a graphical data of a pattern is produced in the following way. FIG. 3 is a flow chart showing a conventional production process for a graphical data. First, a density of a drawing pattern present in a predetermined area is calculated based on a CAD data 68 (step S51). Then, a proximity effect correction is conducted based on the density to calculate an optimum exposure dose (step S52). Thereafter, an exposure dose D.sub.0 is set regardless of whether the drawing pattern is transferred in the cell projection manner or the variably shaped beam manner (step S53). In such a manner, a data for direct drawing 69 is produced. The data for direct drawing 69 is the graphical data of the pattern. Electron beam drawing is performed by use of the data for direct drawing 69 (step S54).
However, there is a difference between an optimum exposure dose for a pattern drawn in the cell projection manner and an optimum exposure dose for a pattern drawn in the variably shaped beam manner. Therefore, when drawings are conducted with the same exposure doses D.sub.0, difference in size arises between patterns drawn in the cell projection manner and the variably shaped beam manner.
FIG. 4 is a schematic drawing showing a pattern formed by a conventional electron beam drawing method. It should be noted that hatching is made on a drawn latent image in FIG. 4. For example, in the case where a cell projection drawing region 71 in which plural repetitions of a basic portion 73 are disposed and a variably shaped beam drawing region 72 in the surrounding thereof are drawn with the same exposure doses D.sub.0, a size L.sub.4 of a pattern 72a drawn in the variably shaped beam manner is narrower than a size L.sub.3 of a pattern 71a drawn in the cell projection manner.
There has been proposed a pattern formation method in which a fluctuation of a line width of a pattern is suppressed in a process step of developing or the like after drawing in Japanese Unexamined Patent Publication (Kokai) No. Hei 5-251318. In a conventional method disclosed in the publication, a fluctuation of a line width is predicted for each pattern forming region, and a dose of charged particles is adjusted based on the prediction.
However, even by this method, if there are a pattern to be drawn in the cell projection manner and a pattern to be drawn in the variably shaped beam manner in a mixed manner, there is a problem that sizes equal in magnitude cannot be achieved between both patterns.